All substances can be classified as elements, compounds or mixtures. Knowing the difference helps us understand chemical formulae and how substances can be separated.
Definitions
- An element contains only one type of atom and cannot be broken down into simpler substances by chemical reactions (e.g. oxygen, copper, carbon). There are over 100 elements in the Periodic Table.
- A compound is formed when two or more elements are chemically combined in a fixed ratio (e.g. water H₂O, carbon dioxide CO₂). It can only be split into its elements by chemical means such as electrolysis.
- A mixture contains two or more substances that are simply mixed, not chemically joined (e.g. air, sea water, brass). It can be separated by physical methods.
Comparing compounds and mixtures
When elements react to form a compound they lose their own properties and combine in a fixed proportion by mass, and energy is usually taken in or given out. In a mixture no reaction happens, so each substance keeps its own properties and can be present in any proportion. Because a mixture is not bonded, it is separated by physical methods such as filtering, evaporation or distillation.
Key idea
A compound has a fixed composition by mass and new properties; a mixture has a variable composition and the substances keep their original properties.
| Compound | Mixture | |
|---|---|---|
| Bonding | chemically bonded | not bonded |
| Composition | fixed | variable |
| Separation | chemical | physical |
Remember
- Elements are listed in the Periodic Table.
- Alloys such as brass are mixtures, not compounds.
- Air is a mixture of gases; water is a compound.